Biomes
Biomes - a major biotic community having well-recognized life forms and a typical
climax species
The terrestrial world is characterized by distinctive assemblages of plant species.
Examples are the temperate forest around Marietta, the tropical rain forests of South
America and Africa, the tundra of northern Canada, the taiga of Siberia. We use plant
species to distinguish biomes because they are immobile and long-lived (at least the
trees). If you used, say geese, to characterize the taiga, what would you do when they
flew south in the winter? The characteristic plants which define each of the biomes
constitute part of what is known as the climax community. The types of plants that
characterize each biome have evolved to the unique climatic conditions in that part of the
world. If such a community is destroyed (by fire, logging, plowing, etc.) - and then left
alone - the biome will regenerate itself over a period of time. It will go through a number
of intermediate stages until it reached the climax stage again. This process is called
succession.
Succession
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orderly succession of communities to a climax community (biome)
sequence of communities --> sere
each transitory community --> seral stage
For instance, in Ohio, a simple description of a sere that would develop on an abandoned
field might be:
grass -> shrubs -> trees -> oak-hickory forest
In this sere, the grass is the pioneer community and the oak-hickory forest is the climax
community. Each step in the sere (grass, shrubs, trees, oak-hickory forest) is individually
known as a seral stage. There are two main types of succession:
1. Primary succession
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begins with bare rock exposed by geologic activity
example sere: rock -> lichen -> moss -> grass -> shrub -> trees -> oak hickory
forest
2. Secondary succession
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begins on soil from which previous community has been removed (by fire,
agriculture, etc.)
old field succession
example sere: grass -> shrub -> trees -> oak hickory forest
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secondary succession can proceed much faster because the soil has been prepared
by the previous community
General trends in succession:
Early seral stages are highly productive but require large inputs of nutrients and also tend
to lose nutrients. Biomass increases, but there is low productivity and fluctuations in
biomass are common. These seral stages are dominated by "weedy" or "r-adapted"
species which reproduce quickly, but often die young. Most of their energy goes into
reproduction. There are relatively few species in early seral stages.
Climax seral stages are much more complex, with many species. They create a favorable
environment for many species. Biomass does not fluctuate, and decomposition rates are
roughly equivalent to new production. Nutrients are cycled efficiently, and rarely leave
the ecosystem. Individual organisms are longer-lived, since they invest more resources in
themselves and less in producing offspring.
Locally, a recently cleared field is an example of an early seral stage. It is colonized by
grasses and other plants that produce many seeds, such as many annuals. These plants
may live only one year, set seed, then die. The organisms in the field will not be able to
cycle all of the nutrients, and many nutrients will run off with rainfall. On the other hand,
the climax forest is characterized by trees, which are long-lived. There are many species,
and each provides living space and food resources for other plants and a host of animals.
Decomposing materials are recycled; few escape though the waters of the forest streams.
If one farms in a rain forest, the community is moved from a climax community to an
artificial community which resembles an early seral stage. The farm field will lose a lot
of nutrients, something that tropical soils do not have in abundance. The land will be
useless for farming in a few years.
This table summarizes the differences between pioneer and climax communities:
Pioneer Community
Climax Community
Harsh environment
favorable environment
biomass increasing
biomass stable
energy consumption inefficient
energy consumption efficient
some nutrient loss
nutrient cycling
low species diversity
high species diversity
fluctuations common
fluctuations less common
r - adapted
K - adapted